Antibodies against double stranded (ds) DNA are a characteristic serologic hallmark of SLE. While it has been demonstrated that anti-dsDNA antibodies play a critical role in the pathogenesis of lupus nephritis, the mechanisms of injury are incompletely understood. Experimental evidence strongly suggests that a least some anti-dsDNA antibodies are pathogenic by virtue of their direct cross reactivity with renal antigen. We recently demonstrated that a pathogenic anti-dsDNA antibody (R4A) binds to a 100 kD protein expressed on the cell surface of a mesangial cell line derived from a lupus-prone MRL-lpr/lpr mouse, and that DNAse treatment of the lysate does not affect binding. Binding was greatly diminished in lysates of mesangial cell line derived from a non-autoimmune mouse, suggesting that antigen expression and/or availability at the level of the target orphan may be a factor in determining susceptibility to lupus nephritis. Following identification of the 100 kD protein bound by R4a as alpha-actinin, the binding of R4A to alpha-actinin was confirmed by Western blot, ELISA, immunofluorescence, and inhibition studies. High titers of anti-alpha-actinin antibodies were found in sera and kidney eluates of lupus mice with nephritis, and in sera of lupus patients. The goals of this proposal are to study if cross-reactivity with alpha-actinin may be an important determinant of the renal pathogenicity of some anti-DNA antibodies, and if the expression of alpha-actinin is genetically regulated and modulated by gender and exposure to cytokines. We will determine if anti-alpha actinin antibodies are pathogenic and if they cross-react with dSDNA, by immunization of mice with alpha actinin and studying the anti alpha-actinin antibody response in normal and autoimmune mice. The molecular basis for the differential levels of antigen display of alpha-actinin between autoimmune and non-autoimmune mouse strains will be studies, and the effects of age, gender, and cytokines known to be present in lupus kidneys on alpha-actinin expression and antibody binding will be determined. Finally, we will identify the epitopes of alpha-actinin that are recognized by pathogenic anti-dsDNA antibodies to understand the generation of anti-alpha actinin antibodies, and determine the potential of alpha-actinin peptides in the treatment of acute lupus nephritis.